Psycholinguists have only recently taken seriously the idea that bilinguals are more representative language users than their monolingual counterparts. The consequence has been a dramatic increase in research on language processing in bilinguals and second language learners. In part, this recent research has focused on how individuals who understand and speak more than a single language negotiate the boundaries of two language systems that may or may not share common features. An important aspect of the comparison of bilingual and monolingual performance is to consider how independently the bilingual's two languages are processed. To date, most research has examined the interactions that exist between a bilingual's two lexicons, with findings indicating a great deal of permeability across language boundaries. One important question that remains is whether this permeability extends to the level of syntactic processing. The research program described here examines the interactions of the bilingual's two languages when bilinguals are making structural decisions about the words they read or hear. Although there is voluminous research examining this question from the perspective of the role of the first language (L1) on the processing of the second language (L2), few studies have examined cross-linguistic interactions in the opposite direction and those that have, have investigated the effects of the L2 on the L1 in bilinguals who have maintained little if any contact with speakers of the L1 or in bilinguals who have negative attitudes toward their L1 (Cook, 2003). To our knowledge, no study to date has investigated the influence of the L2 on the L1 in bilinguals who actively use their two languages daily in a variety of formal and informal contexts, and who value maintenance of the L1. Examining this question has important implications not only for our understanding of how bilinguals manage to negotiate their two languages, but more critically for existing assumptions about the plasticity of cognitive and neural representations. Three studies are proposed involving Spanish-English bilinguals. Study 1 aims at examining the presence of cross-language interactions when bilinguals parse syntactically ambiguous relative clauses in their L1. The focus is on whether language immersion in the L2 can induce fundamental changes in the way that sentences are processed as language unfolds in time. Study 2 investigates whether changes in the L1 can be triggered in a laboratory setting by overexposing bilinguals to particular structures so that proficient bilinguals who have undergone change in their L1 'move back'. Study 3 investigates whether the purported plasticity that allows bilinguals to switch processing preferences in their L1 extends to other linguistic structures. Here, we propose a series of experiments that ask whether native speakers of Spanish immersed in an English environment exploit their knowledge of presence of grammatical gender in Spanish to interpret noun phrases as speech unfolds. Together, the results of these studies have the potential to lead to significant changes in the conceptualization of the mind with two languages and in current views about the permeability of the first language system.